The present invention relates to planar lightwave circuits. More particularly, the present invention relates to improved arrayed waveguide grating (AWG) devices for wavelength-specific filtering and processing in optical communication systems.
Fiber optic communication systems offer far greater capacity than their electrical counterparts (e.g., twisted pair, or coaxial cable) and are therefore attracting much attention as the number and complexity of bandwidth-intensive applications increase. This ever-increasing need for bandwidth that only fiber can deliver is resulting in the continued, widespread deployment of fiber networks.
Legacy fiber systems have for many years existed primarily as untapped, long-haul, point-to-point links between xe2x80x9ccentral offices.xe2x80x9d But simple market forces are pushing fiber networks beyond central offices and into the more architecturally diverse terrain of xe2x80x9cmetroxe2x80x9d markets.
In existing systems, a single wavelength band carrying a single modulated data stream is transmitted across a single fiber link. Digital time division multiplexing (TDM) of the data stream can be used to accommodate separate, independent data channels over the same wavelength band, but these systems require expensive up/down-conversion of the optical signal to an electrical version for multiplexing/demultiplexing the separate channels. While providing some level of operational channelization, TDM techniques generally do not increase the overall data capacity of a link.
Dense wavelength division multiplexing (DWDM) enables the transmission of multiple, independent wavelength bands across a single fiber, thus providing some channelization and a much greater data capacity. Predictably, this capability has resulted in the requirement to add or drop these wavelength bands along the previously untapped lengths of fiber to provide access to the individual wavelength bands. Optical add/drop multiplexers (OADMs) are employed for this function, enabled by arrayed waveguide grating (AWG) components for filtering and forwarding individual wavelengths. AWGs are in the class of xe2x80x9cintegrated,xe2x80x9d wafer-based optical components. They include an on-chip array of closely spaced waveguides having carefully controlled, differing path lengths which cause constructive phase interference patterns on an optical signal transmitted into the array.
AWGs can be useful in many optical communication applications where wavelength-specific filtering and processing are required. Unlike the legacy TDM systems, AWGs function purely in the optical domain when filtering the independent wavelength bands and thus do not require expensive, electrical up/down conversion. As all-optical wavelength filtering components, AWGs have become attractive for optical communication systems. However, as with any component, many technical and economic factors impact AWGs"" viability in the market.
AWGs are thin, fragile chips with narrow waveguides produced using planar lightwave circuit (PLC) processing techniques. The waveguides can be fabricated by forming (e.g., etching) waveguide core patterns over a substrate and undercladding. A doped glass overcladding (e.g., boro-phosphate silicate glass or BPSG) is then formed over the cores, to complete the waveguide formation. As an xe2x80x9cintegratedxe2x80x9d PLC component in a fiber optic system, the optical signals are usually coupled (e.g., at the chip edge) between input and output fiber optics and the on-chip waveguides, leading to concerns about the device""s end-to-end insertion loss. Individual device insertion loss is an important parameter in optical systems because the collective optical link power budget is usually of paramount importance to designers. Since multiple components may be cascaded in series, along with very long lengths of fiber, unacceptable signal losses may result, and can only be remedied by the use of more expensive optical sources (i.e., lasers) or optical amplification. The end-to-end insertion loss through an AWG component is governed primarily by the fiber interfaces to the chip, as well as the signal paths through the AWG itself.
Crosstalk, or channel isolation, is a ratio of power in one channel to the highest level of power meant for that channel but resident in other channels (i.e., for any other channel the term xe2x80x9cnon-adjacent channel isolationxe2x80x9d is used; and for the adjacent channel the term xe2x80x9cadjacent channel isolationxe2x80x9d is used). Crosstalk arises from imperfections in array fabrication, leading to light scattering and errors in focusing the light into the proper output waveguide.
Another particular concern for PLC waveguides, including those in AWGs, is their sensitivity to stress imbalances, and the impact of stress imbalances on optical performance. These stresses can be induced by external environmental conditions, and/or by the fabrication process itself. Stress-induced birefringence in waveguides leads to unacceptably high polarization dependent loss (PDL). Because the materials used for the waveguide layers are different, with differing properties (e.g., differing coefficients of thermal expansion (CTEs)), intra- and inter-layer stresses exist and will result in high levels of waveguide PDL. The fabrication process, and product packaging against environmental conditions, are both key concerns for managing PDL.
In addition to the above-mentioned technical issues, the commercial issues of mass production of AWGs at reasonable costs must also be considered. To enter the market at a reasonable price point, one must consider the absolute number of acceptable die output from the manufacturing process, as well as the manufacturing xe2x80x9cyield,xe2x80x9d i.e., the ratio of the number of acceptable die to the total number of die on that wafer.
Multiple PLC die are usually patterned on a single wafer, and it is desirable to layout as many die per wafer as possible, because of the fixed expense of processing a single wafer in a single production run. This argues against larger AWG devices, which decrease the number of die per wafer, and thus the number of die per production run.
Larger device sizes can also lead to other adverse performance effects, directly impacting manufacturing yield. Process uniformity across a wafer is usually a concern, but that concern is magnified for larger wafer sizes, and for larger individual die sizes, since it is more difficult to maintain strict process uniformity as surface area of a device increases. The refractive index, etch depths and etch biases are all process-controlled parameters which can vary across a wafer surface. Variation of these parameters within a die area can cause adverse optical effects (e.g., crosstalk). Significant xe2x80x9cmicroxe2x80x9d variance in these parameters over smaller areas of a wafer is not expected, and thus a smaller die size will exhibit greater uniformity over its surface area. But xe2x80x9cmacroxe2x80x9d variances across the wafer may occur, and a larger die size including such variances will cause optical performance degradation in the device within that die. As process technology shifts from six (6) inch wafer size to eight (8) inch wafer sizes, concerns about uniformity increase accordingly.
Various techniques have been proposed to address each of the above concerns individually. However, most proposals suffer from a fundamental weakness: they lack any significant consideration of the complex interaction between the process technology used to fabricate the device structures, the resultant device structures, the process economics, and optimizations to the process and structures based not only on repeated performance testing, but on environmental and reliability concerns. Moreover, the known proposals usually contain only vague design guidelines, without providing any device specifics, as a function of processes and economics, for suitable AWGs.
What is required, therefore, are improved arrayed waveguide gratings (AWGs), and methods for their fabrication, having the desired performance and reliability characteristics, especially in the loss, isolation, PDL and packaging areas discussed above, while also considering the economic issues for successful fabrication and commercialization.
These requirements are met, and further advantages are provided, by the present invention which in one aspect is a planar lightwave circuit, and methods for its fabrication and use, having an arrayed waveguide grating (AWG). The AWG has a plurality of input and output waveguides; a plurality of at least partially curved array waveguides with respective length differences for imparting respective phase delays on respective optical energy transmitted therein; an input planar waveguide region coupled between the input waveguides and array waveguides, for receiving input optical energy from at least one input waveguide and distributing the input energy to the array waveguides; and an output planar waveguide region coupled between the array waveguides and at least one output waveguide, for receiving the respectively phase delayed energy from the array waveguides and distributing said energy to the at least one output waveguide.
The present inventors have carefully balanced the above-stated requirements of optical performance, device size, and reliability, and have invented a unique, compact AWG having features such as:
Optimal widths and spacings of waveguides (especially the array and output waveguides) along the planar waveguide region facets, which are largely determinative of AWG size and optical performance;
Optimal waveguide cross-section (e.g., width and height) for optical performance and alignment to fiber cores;
Modified index of refraction difference (0.78%) between the waveguide core and cladding regions, as an independent variable to ensure proper optical energy confinement and therefore optical performance; and
Optimal array waveguide numbers, lengths, path length differences, and free spectral range for the wavelength bands and band spacings of interest.
These features, especially when combined with advanced fiber array attachment, passivation and packaging techniques, advantageously result in a compact high-yield, high-performance AWGs (both gaussian and flattop versions), about 40 of which can be accommodated on a single 200 mm (i.e., 8 inch) wafer.